She is best remembered for portraying Major Margaret "Hot Lips" Houlihan in Robert Altman's acclaimed 1970 film M*A*S*H, in which she co-starred with fellow Star Trek performers Rene Auberjonois, John Schuck and Fred Williamson. Later that same year, she again appeared with Auberjonois and Schuck in Altman's Brewster McCloud, also starring William Windom and Bert Remsen. She would co-star with Auberjonois yet again in the 1976 comedy The Big Bus, which also featured Vic Tayback.

Kellerman's sensual voice is often heard in several TV commercials. She also provided voices for the animated features The Mouse and His Child (1977) and Happily Ever After (1990, with Malcolm McDowell and Frank Welker). Kellerman also lent her voice to the director's cut of Brian Hegeland's Payback (1999), playing "Bronson," the faceless boss of a New York City crime syndicate. Other Trek performers who appeared in the film included Gregg Henry, John Glover, and Jeff Imada.